Various devices and methods for pressing a liquid-core cast product are known from the prior art, and this operation is called “soft reduction”.
This technology is particularly common within the area of slabs, which are characterized by being much wider than they are thick.
The skin of the product begins forming in the crystallizer due to the progressive cooling to which the product is subjected. As the product travels downstream dragged by the straightening units, it is subjected to continuous direct and indirect cooling operations and this results in an increase of the thickness of the skin, resulting from the subtraction of heat from the core of the product performed by the cooling system.
The product is cast from the tundish into the crystallizer and starts its descent towards the extraction area downstream, being cooled and contained by the containment rolls. The thickness of the product skin increases as the product descends and cools off, until there is the spontaneous joining of the skin in the so-called “metallurgical cone”, at which point the complete solidification of the product is achieved.
The process for forming the skin generally is influenced by various parameters, in particular by the steel grade of the cast product, by the heat exchange undergone by the cast product during casting, by the casting speed and by the dimensions of the product itself.
It is necessary that the complete solidification of the product occurs so as to preserve the inner quality thereof: indeed, as the product solidifies, there is a decrease of the volume occupied by the liquid fraction which initially occupies a larger volume with respect to the solid fraction. This volumetric difference does not affect the product very much in the first part of the casting curve, since the liquid fraction volume lost during solidification is replaced by the liquid further upstream pushed downstream by the ferrostatic pressure. However, in proximity of the vertex of the metallurgical cone, the solid component and the liquid component are no longer well distinguished from each other, thus causing the so-called “mushy zone”.
From a microscopic point of view, the appearance of the skin of the product in contact with the liquid core has a series of crystalline branches called dendrites, which when the skin is about to be joined, tend to intersect with one another, thus forming a barrier for the inlet of the liquid above, preventing filling with new liquid in the areas subjected to a decrease of the volume of liquid fraction due to the solidification, and causing the formation of undesired porosity in the inner structure of the product.
A further problem generated in this solidification step is that of macro segregations: as the product solidifies, the dendrites extend and tend to bring the alloy elements (e.g. carbon, sulfur, etc.) towards the liquid core of the product. This phenomenon causes a difference in the chemical composition along the section of the product. These migrations of alloy elements cause undesired differences of the mechanical properties, thermal properties, etc. between the various areas of the product, while a product having uniform structure and properties is instead desirable.
In order to obviate these drawbacks, the soft reduction treatment was developed, which provides the controlled pressing of the cast metal products, e.g. slabs or blumes or billets, wherein the cast metal product is subjected to an action of reducing the thickness while the core is still liquid or partially liquid in an area downstream of the ingot-mold, thus obtaining a less thick product with respect to the cast one at the outlet of the continuous casting machine.
The main advantage of reducing the thickness of the liquid or partially liquid core is to obtain an improvement of the solidification structure together with an improved inner quality of the cast product.
In order to be effective, the soft reduction should occur with a continuous and controlled reduction of the thickness of the cast product up to when it contains therein a liquid or partially liquid core, which may be obtained with a substantially conical reduction profile of the stretch of cast product involved.
The most common soft reduction devices provide pressing the product by means of pairs of opposite rolls: the pressing force here is therefore applied with equal intensity and opposite direction, thus causing a decrease of the thickness of the product and an extension thereof (called “bulging”).
This soft reduction treatment is commonly used in the field of continuous slab casting since the widening of the side faces is not such as to seriously affect the finished product which, once the curved sides have been conveniently trimmed, will be ready for rolling or other successive operations.
With regards instead to products having rectangular or square section, the soft reduction is to be performed more carefully, since an excessive curving would cause an excessive deformation of the products which would then be difficult to process.
This problem is felt even more for round-section products since keeping the shape is essential for processing and selling the product on the market: indeed, by using only two rolls which press the product in opposite directions to close the liquid core thereof, there is a risk of excessive oval deformation of the section of the product. In an attempt to correct this ovalization, another deformation may be generated, made with rolls shaped so as to obtain a round section with a smaller section. However, this further deformation, which requires at least two passes, does not always decrease the section of the product and at the same time keep it perfectly round. Indeed, further forming passes are often required downstream for the further definition of the round geometry.
A partial solution to this problem provides eliminating the first pressing step by directly casting an elliptical-section product which in the next soft reduction step is deformed into a round shape by two parallel shaped rolls.
However, the deformation operation to pass from the elliptical section to the round section—in particular, the more the ellipse is pressed—results in tensions in the core of the product which may damage the inner quality thereof.
Thus, the need is felt to provide a soft reduction device for round-section cast products which allows to overcome the aforesaid drawbacks.